84 lines
2.6 KiB
Python
84 lines
2.6 KiB
Python
# SPDX-License-Identifier: MIT
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from __future__ import annotations
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import random
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import time
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from typing import Callable, Generic, Literal, TypeVar, Union, overload
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T = TypeVar("T", bool, Literal[True], Literal[False])
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__all__ = ("ExponentialBackoff",)
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class ExponentialBackoff(Generic[T]):
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"""An implementation of the exponential backoff algorithm
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Provides a convenient interface to implement an exponential backoff
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for reconnecting or retrying transmissions in a distributed network.
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Once instantiated, the delay method will return the next interval to
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wait for when retrying a connection or transmission. The maximum
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delay increases exponentially with each retry up to a maximum of
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2^10 * base, and is reset if no more attempts are needed in a period
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of 2^11 * base seconds.
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Parameters
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----------
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base: :class:`int`
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The base delay in seconds. The first retry-delay will be up to
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this many seconds.
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integral: :class:`bool`
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Set to ``True`` if whole periods of base is desirable, otherwise any
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number in between may be returned.
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"""
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def __init__(self, base: int = 1, *, integral: T = False) -> None:
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self._base: int = base
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self._exp: int = 0
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self._max: int = 10
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self._reset_time: int = base * 2**11
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self._last_invocation: float = time.monotonic()
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# Use our own random instance to avoid messing with global one
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rand = random.Random()
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rand.seed()
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self._randfunc: Callable[..., Union[int, float]] = (
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rand.randrange if integral else rand.uniform
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)
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@overload
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def delay(self: ExponentialBackoff[Literal[False]]) -> float:
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...
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@overload
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def delay(self: ExponentialBackoff[Literal[True]]) -> int:
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...
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@overload
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def delay(self: ExponentialBackoff[bool]) -> Union[int, float]:
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...
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def delay(self) -> Union[int, float]:
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"""Compute the next delay
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Returns the next delay to wait according to the exponential
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backoff algorithm. This is a value between 0 and base * 2^exp
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where exponent starts off at 1 and is incremented at every
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invocation of this method up to a maximum of 10.
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If a period of more than base * 2^11 has passed since the last
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retry, the exponent is reset to 1.
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"""
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invocation = time.monotonic()
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interval = invocation - self._last_invocation
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self._last_invocation = invocation
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if interval > self._reset_time:
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self._exp = 0
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self._exp = min(self._exp + 1, self._max)
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return self._randfunc(0, self._base * 2**self._exp)
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